1. Field of the Invention
The present invention relates to a developing device, and an image forming apparatus.
2. Description of the Related Art
In electrophotographic image forming apparatuses, such as copiers, printers, and facsimile machines, a developing device is used that visualizes or develops an electrostatic latent image on the surface of an image carrier, such as a photosensitive member, with two-component developer containing toner and carrier. In such a developing device, it is required to maintain a constant mixing ratio of carrier and toner of the developer and also appropriately control the charge amount of toner. In particular, when solid images, which consume a large amount of toner, are successively output, the large amount of supplied toner is left on a developing sleeve without being mixed with developer, thereby causing problems, such as toner flying and scumming (attachment of developer onto a non-image portion of a photosensitive member).
To overcome these problems, for example, Japanese Patent Application Laid-Open No. H10-177203 has proposed a conventional technology in which, before being supplied, toner is preliminarily agitated with a small amount of developer to be in a charging state to some degree, and then fed to an agitating unit.
Because the charge amount of toner is decreased with time, agitation performed twice is inefficient. In addition, two agitating portions are required, which increases the size of the apparatus. To overcome these drawbacks, Japanese Patent No. 3349286 discloses a developing device with a simple structure that controls the toner density and charge amount of the developer fed to the developing sleeve always within an appropriate range. The conventional developing device includes developing means that provides a developer with a toner and a carrier mixed therein to the image carrier and a developer optimizing unit for optimizing the developer. The developer optimizing unit is arranged separately from the developing means, and includes developer optimizing means having incorporated therein at least toner resupplying means and an agitating member for the toner and the carrier, and developer transporting and circulating means that transports the developer to the developing means and the developer optimizing means.
In the conventional developing device, toner is resupplied to a developer agitating unit to be mixed with a developer with a decreased toner density. However, because of the low specific gravity, the toner flies in the air or is left unmixed on the developer. Therefore, an agitating time may be long to make the toner density of the developer uniform. Also, when the developer agitating unit is resupplied with toner, there is a possibility that the resupplied toner may be mixed with the developer with an ideal toner density already achieved by agitation. Therefore, there is room for reconsidering agitation efficiency.
In a developer container containing two-component developer for the developing device, the toner is consumed through a developing operation. Therefore, the developing device is refilled with new toner, and the toner is conveyed together with carrier for dispersion. At this time, the toner and carrier are triboelectrically charged to cause an electric charge at the developer, thereby charging the toner. If the resupplied toner is not sufficiently dispersed or charged until the toner is supplied to the development roller, flying of charge-failed toner from the developing device may occur, thereby causing scumming on the image.
In particular, at a high-speed machine, a circulation speed of the developer in the developing device is fast. Therefore, a time for sufficiently charging the toner cannot be ensured. To get around this, to sufficiently charge the toner, the capacity of the developer is increased so that the resupplied toner is easy to be dispersed. However, the size of the developing device is increased, resulting in upsizing of the machine body. In view of this, Japanese Utility-Model Publication No. H5-21082 has proposed a conventional technology in which, in the developing device using two-component developer, developer agitating means is provided inside a photosensitive drum, and is connected to a developing unit with a pipe for circulation of the developer. Japanese Patent Application Laid-Open No. H4-198966 has proposed another conventional technology in which a developing unit and a developer agitating unit are separated in a developing device and these developing unit and developer agitating unit are connected together by developer circulating means. Japanese Patent Application Laid-Open No. H7-225515 has proposed another conventional technology in which a developing unit and a developer agitating unit in a developing device are connected together by developer circulating means and a screw pump is used for transporting the developer.
With the conventional technologies, a charge amount is adjusted in advance to a desired value by agitating toner and carrier outside a developing device. However, the charge amount of toner varies depending on various factors, and the charge amount may be different even with the same agitating conditions. For example, the factors include the surrounding environment (temperature and humidity), a decrease in charging capability of the carrier with time, and a change in toner charge amount due to an output image area, i.e., a difference in charge amount due to toner retention time, or the like. Moreover, due to the amount of toner newly resupplied, the charge amount may be varied, because mixing and agitating takes time if the amount of toner is large.
Therefore, in the conventional technologies, the charge amount is not constant depending on conditions, thereby affecting image quality. One scheme for solving such problems is to control an agitating force at a developer container outside the developing device to obtain an appropriate charge amount. However, agitation may exert stress on the developer, thereby accelerating deterioration of the toner and carrier.